Razor knife with frangible sheath



FIG. 1 is a front elevational view of a razor knife with frangible sheath showing my new design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a left side elevational view thereof; and

FIG. 4 is a top plan view thereof. 

The ornamental design for a razor knife with frangible sheath, as shown and described. 